3 Answers
3

At a high level ... I hope someone else will explain in more detail and with the underlying theory.

Loops have to do with magnetic coupling / interference / compatibility between two circuits.

With a larger loop, more magnetic field lines will fit inside the loop, so the magnetic coupling between this loop on your PCB and an interfering outside magnetic field can be better. If you make your loop infinitely small, then no field lines fit inside your loop, hence no magnetic coupling is possible.

Why does the size of the loop decide the size of the antenna?
–
richieqianleJul 9 '14 at 8:09

@richieqianle because at a given 'flux' density or field line density, there are more field lines enclosed in a larger loop than in a smaller loop. The amount of flux inside the loop is what really counts.
–
jippieJul 9 '14 at 9:01

Thanks! I mean, why does the size of the loop decide how much field lines will be enclosed, instead of the size of, say the board?
–
richieqianleJul 14 '14 at 7:19

@richieqianle It is easier to determine the surface (and with that its effective number of field lines) inside a loop than the infinitely large surface outside the loop (where numerous different fields will largely cancel each other out). The size of the board doesn't matter at all as that is just an arbitrary size, if you change the size of the board, you won't change the EM field, nor the loop.
–
jippieJul 14 '14 at 8:29

It's all about Faraday's Law when it comes to picking up interference (one half of the story).

Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a succinct summary of the ways a voltage (or emf) may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field.

All taken from this website. Basically the bigger the area of the coil the more flux it can receive and the greater the voltage it can produce across its terminals. For the creation of a mag field see this (taken from same site): -

Basically, the bigger the area of the coil, the bigger the magnetic field generated hence the greater the interference received.

Thanks Andy! It explains well that larger loop reads larger interference from the environment. But why do larger loops produce more EMI?
–
richieqianleJul 14 '14 at 7:18

1

@richieqianle - a larger loop area has more inductance and inductance is defined as total flux per amp therefore, for a given current (and a larger loop area), more interfering flux is generated.
–
Andy akaJul 14 '14 at 7:26

The answer that's most easy to understand is that a big antenna radiates more than a small antenna. And that is what the loop is. Also, as you make the loop (antenna) smaller, you raise the frequency at which it will (want to) radiate. At some point it is so small that the radiation is greatly reduced.